1. Field of the Invention
The present invention relates to amplifiers and more particularly to transconductance amplifiers, i.e., amplifiers which provide a current output in response to a voltage input. Still more particularly, the present invention relates to an amplifier for driving a head positioner motor to position the magnetic head of a disk drive system with respect to concentric data tracks on a disk.
2. Description of the Prior Art
Disk drive systems are widely employed for memory storage in computer systems. Often, the disk drive system is manufactured by a separate manufacturer and supplied as a plug in device to the computer manufacturer. The disk drive includes a motor for positioning the magnetic heads of the system with respect to data tracks on the disks. The amplifier for driving the motor is typically supplied by a voltage supply which is a part of the main computer system rather than the disk drive system. In order to reduce the cost to the computer manufacturer, it is desirable to provide an amplifier which operates with a single power supply. However, the amplifier must be able to provide current in two directions, since the motor drives the heads bidirectionally. The amplifiers employed in such systems therefore typically include a bridge network for providing two current output paths from a single supply voltage.
Transconductance amplifiers are desirable in servo designs for various reasons. Transconductance amplifiers are generally more stable than voltage amplifiers, provide better drive characteristics, and have fewer performance variables associated with them. However, transconductance amplifiers typically are not as rugged as some other types of amplifiers.
Bridge networks typically include four transistor devices interconnected in an H configuration, with one transistor being located in each leg of the H, and the transistors being interconnected by the bridge between the legs. Typically, the upper transistors serve as switches and the lower transistors are operated in the active mode to control the amount of current provided by the bridge. Since the upper transistors are operated at as switches, they do not dissipate any power, and the entire power dissipation must be provided by the lower transistors. This increases the cost of the bridge network.
In controlling a bridge network, current flows from a power supply through one of the upper transistors, across the bridge connection, and through the lower transistor. A problem encountered in various bridge networks is that of "feedthrough", in which both the upper and lower transistors on the same side of the H pattern conduct, with the result being that the bridge fails to provide the desired current.
In addition to the power dissipation and feedthrough problems associated with prior art bridge designs, the switching action of the transistors can generate radio frequency interference (RFI) which can interfere with the operation of the magnetic heads of the disk drive.